(1) Field of the Invention
The present invention relates to the map display apparatus which displays a map on a screen, in particular, to a map display apparatus which generates a three-dimensional image from the electronized map data, and displays a map on the screen.
(2) Description of the Related Art
Conventionally, a map display apparatus which generates and displays a three-dimensional image from electronized map data has been used, and has been applied to, for example, a car navigation apparatus, a map display software for personal computer, and the like (for example, refer to Japanese Laid-Open Patent publication No. H9-281889).
In the above mentioned map display apparatus, in order to display, on a two-dimensional screen, an object such as a building and a high bridge which have been defined in a three-dimensional space, a three-dimensional coordinate is transformed into a two-dimensional coordinate. Here, the coordinate system, as shown in FIG. 1A, in which the center of each three-dimensional object is the origin point is called a “local coordinate system”. The coordinate space, as shown in FIG. 1B, which includes all of the above mentioned objects is called a “global coordinate system”. The coordinate space, as shown in FIG. 1C, which can be viewed when facing a certain direction (called z axis direction) from a certain point (called the origin point) of the global coordinate system is called a “view coordinate system”. The coordinate system is called a “screen coordinate system”, the coordinate system being two-dimensionally perspective projected so as to display, on a screen, the part included in the field of view from the viewpoint in the view coordinate system. Also, the transformation from the local coordinate system into the global coordinate system is called a “local-to-global transformation”. The transformation from the global coordinate system into the view coordinate system is called a “view transformation”. The transformation from the view coordinate system into the screen coordinate system is called a “projection-transformation”. The transformation for adjusting the projection-transformed coordinate to the proper size for the final display area on the screen of the map display apparatus is called a “viewport-transformation”. In other words, in the map display apparatus, in order to display the object defined in the three-dimensional space, on the two-dimensional screen of the map display apparatus, the transformation is executed according to need, from the coordinate system for which each object is defined into the screen coordinate system.
When a three-dimensional space is displayed on a two-dimensional screen, in the same way as actually viewed with human eyes, a close object is displayed large, and a far object is displayed small. FIG. 2A, FIG. 2B and FIG. 2C are schematic diagrams for explaining perspective distortion: FIG. 2A is a side view of an object (building); FIG. 2B is a front view of the object (building); and FIG. 2C is a front view of the whole screen. For example, as shown in FIG. 2A, in the case where the top part of the building is close (distance “a”) to the viewpoint position (origin point), and the bottom part of the building is far (distance “b”) from the viewpoint position (origin point), as shown in FIG. 2B, the top part becomes large (distance “c”), and the bottom part becomes small (distance “d”). Thus, depending on the perspective from the viewpoint, so-called perspective distortion occurs, and as shown in FIG. 2B, lean occurs in the border line of the vertical direction of the building (vertical direction of the screen). The farther a building exists from the view line, the larger the effect of the above mentioned perspective distortion becomes.
In the above mentioned conventional map display apparatus, as shown in FIG. 2C, the position of the fixation point W in the view coordinate system often exists around the screen center. The closer to the end of the screen a building is rendered (in particular, rendered on the front side), the more the building deviates from the view line. Due to the effect of the perspective distortion, the building cannot be rendered straight on the “y” axis (vertical direction of the screen) of the screen coordinate, and is leaned. Thus, in an apparatus such as a car navigation apparatus which uses a low resolution display, regarding the building on the screen end of the display apparatus, as the border line of the building in the vertical direction is leaned, as shown in FIG. 3A, jaggies appear, and a straight border line in the vertical direction as shown in FIG. 3C cannot be displayed beautifully.
As counter measures for the above mentioned problem of jaggies, first, increasing the display resolution is conceivable. However, to achieve this, improved performance of the hardware is necessary. Also, as another counter measure, executing an anti-aliasing process is conceivable. This anti-aliasing process is a process to complement, using half tone, the space between the border line in the vertical direction of the leaned building and the background, and to make the border line appear smooth. However, to achieve this, improved processing performance of CPU is necessary. Thus, in the status quo, it is difficult to use the above mentioned counter measures for the apparatus such as a car navigation apparatus which has little hardware resource.
By the way, without executing coordinate transformations for all of the vertexes forming the building as described above, a method for artificially generating a solid by providing height to the vertexes forming the bottom plane of the building is conceivable. According to the above mentioned method, for example, in the case of a solid building, without executing coordinate transformations for the eight vertexes A to H forming the building as shown in FIG. 4A, a three-dimensional image is rendered by providing the same height to the four vertexes I to L and artificially generating a solid. Thus, the building is rendered straight on the “y” axis (vertical direction of the screen) of the screen coordinate. However, for example, when the viewpoint is set closer to the building, in the case of the method using the coordinate transformation, the building is coordinate transformed into the form viewed from the viewpoint, and the top plane of the building cannot be viewed as shown in FIG. 5A. On the other hand, in the case of the method for artificially generating a solid, as shown in FIG. 5B, although the viewpoint is looking up the building, the top plane of the building can be viewed, and an unnatural image is rendered.